1 Introduction

The feeding habit of the snail in the wild, part of which includes feeding on litter and dead animals predisposes it to the potential of accumulation of pollutants like heavy metals in their tissues, thus it may provide important links in the transfer of chemicals from vegetation or litter to carnivores. Such transfer along food chains is one of the important aspects of ecotoxicology (Monu et al, 2008).

Gastropods and bivalve molluscs accumulate metals in their tissues in proportion to the degree of environmental contamination and can be used as indicators of marine pollution (Chiara Copat et al, 2013). They are very appropriate as environmental monitors’ in-situ because they are sedentary, abundant, of relative longevity, large, easily collected and weighed. Patterns of accumulation of one specific element show a remarkable consistency when different sites are compared (Ademoroti, 1996). Some species contain high amounts, whereas others contain only small quantities. In the case of Cadmium, highest concentrations are always found in isopods, earthworms, spider and pseudo scorpion, whereas collembolans and carabid beetles are always among species with lowest concentration (Ademoroti, 1996).

The term “heavy metals” is commonly adopted as a group name for metals and the metalloids, which are associated with pollution and toxicity, but also include some elements, which are essential for living organisms at low concentrations. Heavy metals have density greater than 5 gcm^-3. There are many of such metals namely; Aluminium, Arsenic, Beryllium, Bismuth, Cadmium, Chromium, Cobalt, Copper, Lead, Mercury, Nickel, Selenium, Strontium, Thallium, Tin, Titanium and Zinc. These metals or their compounds may be discharged from industries, farmlands, municipal urban run-offs etc. into surface waters to cause pollution.

Pollution of the ecosystem by toxic metals during man’s activities poses serious threat because:

· Metals are non-degradable and are persistent in the ecosystem.

· Physical, chemical and biological processes may combine under certain circumstances to concentrate metals rather than dilute them.

· The societal impact of a contaminated system is substantial.

Warren (1981) noted that various activities by man in recent years have increased the quantity and distribution of heavy metals in the atmosphere, land and water bodies. The extent of this widespread but generally diffused contamination has caused concern about its possible hazards on plants, animals and human beings (Herros et al, 2008) Heavy metals are passed on to man by food chain and the cumulative effects of these metals, most of which are toxic, are adverse generally. Examples are:

Lead (Pb):- Lead compounds [PbO and (C₂H₅)₄Pb] are toxic and have resulted in poisoning of workers from misuse and overexposure. There are many adverse effect of Pb as its poisoning can cause brain damage which result in poor memory, sluggishness and restlessness. Renal damage has also been caused by heavy exposure to Pb. Fertility problem and female menstrual disturbances have been caused by Pb poisoning.

Copper (Cu):- Copper metal is insoluble, therefore, copper mostly enters the environment as Cu salts such as CuCl₂, CuSO₄ and CuNO₃. Allegic dermatitis may occur from direct exposure to copper salt or dust but this is not common. Copper is a benign agent to human under normal conditions. Heredity copper toxicosis in human known as “Wilson’s Disease” which results from copper homeostasis, deficient and toxic levels accumulate in several tissues. In Wilson’s disease, copper accumulation in the liver affects infant growth. Evidence of histology damage can be seen in early infancy, (Nriagu, 1979). Copper has also been reported to cause skin irritation and conjunctures, which may be in an allergic basis. Cuprous oxide is irritating to the eyes and upper respiratory tract.

Cadmium (Cd):- Cadmium is one of the rare elements in the earth crust [0.15-0.2ppm]. However, it is rather widely distributed and is found in shale, igneous rock, coal, limestone, soils and oceanic sediment. Cadmium has no biological function and is highly toxic to plants and animals. The major hazard to human health from cadmium is its chronic accumulation in the kidney where it can cause dysfunction if the concentration in the cortex exceeds 200mg/kg fresh weight. Cardiovascular disease has been related to ingestion and inhalation of Cadmium (Table 1).

For any element or compound to accumulate in any organism, the exposure must continue over an extended period of time, particularly if the exposure is through food chain, (Mohamed and Ahmed, 2006). This implies that the given element or compound must be resistant to available breakdown process. In this case, it persists in the organism as long as it lives. In an aquatic environment, absorption process appears to be a significant step in bio-accumulation, (Shaohua Wu et al., 2011). Rice and Sikka (1973) and also Munson et al. (1976) independently found that the size of an organism is a factor in determining bio-accumulation. Tinsley (1979) observed that filamentous algae that grow in long stings have very large surface area and show greater tendency to accumulate compounds from aquatic environment. Also, mist organism contains significant fat deposits, which help accumulation of heavy metals in their tissues.

Different organisms have different metabolic rates and different food amounts and requirements to sustain these rates. Hence, rates of food intake may also be a significant factor. Organisms that require high amount of food intake may tend to accumulate high levels of a particular compound provided they do not compensate by more active excretion, (Qiao-Mei Ru et al., 2013).

Research studies have confirmed the usefulness of snails in the treatment of some human diseases as a result of certain substances found in snails. Snail meat is low in sodium, fat and cholesterol but contains high amount of protein, iron and calcium hence it is useful in the treatment of hypertension, arteriosclerosis, high blood pressure and other fat related diseases. It also helps to reduce haemorrhoids and constipation. The fluid from edible snails can be extracted and has been scientifically tested and found to contain over 90% natural anti-hypertensive substance. Carbonized snail powder prepared from snail fluid when mixed with coconut milk serves as a potential cure for many heart diseases. The iron deficiency anaemia that was observed to be common in tropical countries (Oyenuga, 1968) could be treated by consuming snails as they constitute a rich source of iron.

Research studies have confirmed the usefulness of snails in the treatment of some human diseases as a result of certain substances found in snails. Snail meat is low in sodium, fat and cholesterol but contains high amount of protein, iron and calcium hence it is useful in the treatment of hypertension, arteriosclerosis, high blood pressure and other fat related diseases. It also helps to reduce haemorrhoids and constipation. The fluid from edible snails can be extracted and has been scientifically tested and found to contain over 90% natural anti-hypertensive substance. Carbonized snail powder prepared from snail fluid when mixed with coconut milk serves as a potential cure for many heart diseases. The iron deficiency anaemia that was observed to be common in tropical countries (Oyenuga, 1968) could be treated by consuming snails as they constitute a rich source of iron.

It is therefore imperative that which should be concern about the chemical composition of this all important animal, hence the screening of snails bought in the open market of Abeokuta for possible health risk in terms of possible presence of heavy metals.

2 Materials and Method

2.1 Sampling procedure

Snail samples were bought from 8 markets in Abeokuta. Four samples were bought from each market to make 32 samples in all. The samples bought were of about the same size in order to ensure uniformity and representativeness. The markets covered and their codes are shown in Table 2.

Table 2 Markets and Codes for sampling areas

2.2 Sample preservation

The snails were removed from their shells and stored in the freezer i.e. deep freezing. This was done to preserve the integrity of the organic matter in the snails.

2.3 Cleaning of glassware

Glassware thoroughly washed with solution of detergent and then rinsed with distilled water. They were later allowed to drain. A small quantity of concentrated HNO₃ was introduced into the glassware’s and placed on the heating mantle. The fumes produced from the heating of the acid condensed on the walls of the glassware’s to destroy the organic contaminants.

The glassware’s were removed from the heating mantle and allowed to cool for some minutes and then rinsed with distilled demineralised water. The glassware’s were later placed back on the heating mantle to dry. This completed the cleaning of the glassware’s and the procedure was repeated and after the use of the glassware’s.

2.4 Sample preparation and digestion

This was done by weighing 1.5g of the snail samples and transferring into a conical flask. HNO₃-H₂SO₄ wet digestion method was used for decomposition of the organic matter present in the snail samples. To the sample in the conical flask was added 5ml of concentrated HNO₃ followed by 10ml of concentrated H₂SO₄. This was then placed on the heating mantle at a temperature of 100^oC and Nitrogen (IV) oxide fumes were given off. Heating was continued until a clear solution was obtained indicating that digestion was complete. The conical flask was then allowed to cool for some minutes after which the clear solution was transferred into a measuring cylinder and made up to 50ml with distilled water. The digested samples were then transferred into digestion bottles and subsequently labelled.

2.5 Sample analysis

The digestion bottles containing the digested samples were taken to the Chemistry Department Laboratory for qualitative and quantitative analysis to identify and quantify the heavy metals of interest using the Atomic Absorption Spectrometer (AAS).

3 Result and Discussion

3.1 Lead

The level of lead in snail flesh from the 8 markets ranged from (4.15 – 7.93 mg/kg). The level of lead was highest in Lafenwa market, a concentration range of (7.88 – 7.99) and lowest value was obtained at Omida market (4.06 – 4.22 mg/kg). The result of the reported work of Ogbona (1992), showed that fleshy tissue of giant land snail had Pb concentration in the range of (0.34 – 4.0 mg/kg). The result obtained in this study showed an increase in the level of Pb in the snail samples, though the history of sampling point is a great contributor to the reported level of Pb.

The relatively high level of Pb observed for Lafenwa market may be as a result of industrial and agricultural activities along with bioaccumulation of metal in the food chain. Furthermore, the mean value of Pb obtained from all the markets was given as 5.64mg/kg, which is below the toxic level when compared to FAO/WHO standard of 15mg/day for an adult.

3.2 Copper

The level of copper ranged from (0.339 – 0.719 mg/kg). The highest level was found at Lafenwa market, a concentration range of (0.711 – 0.728 mg/kg) and the lowest concentration at Omida market (0.336 – 0.341 mg/kg). The mean value of copper for all the markets was given as 0.492mg/kg which is below the toxic level using the FAO/WHO standard for total body copper for an adult which is about 1-5 mg/day. The high level at Lafenwa market could be attributed to activities of man from the snail source.

3.3 Cadmium

The level of Cd in snails were rather too low to be detected, this could be attributed to low level of cadmium in the environment which most of the snails originated from. However, values were obtained at Elega, Iberekodo and Lafenwa markets with mean concentrations at 0.03, 0.03 and 0.06 mg/kg respectively (Table 3).

4 Conclusion

The level of heavy metals (Pb, Cu and Cd) from 8 markets in Abeokuta revealed that Lafenwa market had the highest concentration of heavy metals in snails while the lowest concentrations were found in Omida market.

The heavy metals followed the trend of Pb>Cu>Cd. From the results obtained, when compared to other literatures shows that, though the snails were contaminated, but still below the toxic level. Thus, the need for monitoring of levels of heavy metals in edible snails on a regular basis given the importance attached to this highly nutritious and popular species.

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